This invention relates to thin film solar cells and more specifically to a PIN amorphous silicon solar cell in which a nitrogen compensated intrinsic inter-layer is provided between the front face P type layer and the intrinsic layer.
Silicon photovoltaic, or solar, cells have been manufactured in single crystal, polycrystalline, and thin film forms. It is believed that the thin film silicon cell will eventually displace the other types due to the dramatic reduction in qunatity of silicon used per watt of output power. Much effort has been extended in improving the reliability and efficiency of such thin film solar cells. Efficiency improvements reduce the cell area required per watt of output and thereby reduce the cost of materials, including silicon, used per watt of output power.
For example, U.S. Pat. Nos. 4,398,054 and 4,415,760 issued to Madan on Aug. 9, 1983 and Nov. 15, 1983 teach the use of a thin insulating layer between the intrinsic amorphous silicon layer and an N type layer forming the light receiving face of the cell. This insulating layer is preferably 3 nanometers thick and formed of materials such as Si.sub.3 N.sub.4, SiO.sub.2, BN, Ta.sub.2 O.sub.5, Nb.sub.2 O.sub.5, and the like. Madan teaches that such an insulating layer increases the quantum efficiency of the solar cell in the blue region of the solar spectrum.
U.S. Pat. No. 4,387,387 issued to Yamazaki on June 7, 1983 likewise teaches improvements in thin film silicon solar cells. The particular improvement is a thin nitride layer formed between the light receiving face of the solar cell structure and the transparent conductive contact normally applied to such light receiving face. Yamazaki teaches that this nitride layer inhibits the passage of undesirable impurities into the semiconductor layer and that such undesirable impurities would reduce the efficiency of the solar cell.
In both Madan and Yamazaki the additional layer of, for example, Si.sub.3 N.sub.4 is a true insulator having resistivity. As a result it is essential in using such materials that layer thickness be very carefully controlled to avoid increasing device resistance and thereby reducing device efficiency.